Draw lines from the phone symbols to the black and white circles, so that each phone is connected to one black and one white circle, and each circle is used exactly once. The lines run horizontally and vertically, and each empty cell is used by exactly one line. If a circle contains a number, that number indicates how often the line from the respective phone symbol to this circle makes a turn. For empty circles the number of turns is unknown.
Grid control: To draw a line press the mouse button at a start cell and move mouse to another cell holding the button. More details about grid control are at the help page.